Antibodies are useful as therapeutic agents, diagnostic agents, or reagents for various diseases. Many kinds of antibodies have been isolated to date. General methods for producing antibodies comprise the steps of administering antigens to mammals such as mice; and obtaining antibodies derived from the serum of these animals. However, subject antibodies are not always obtained efficiently by antibody production methods, as in the following cases, for example:
when a small quantity of antigen is used to immunize mammals; or
when an insufficiently purified antigen is used to immunize mammals.
Therefore, when immunizing, it is desirable to prepare a large quantity of a sufficiently purified antigen. Practically, however, many antigens are difficult to purify or to sufficiently prepare. Thus, the step of antigen preparation has often prevented antibody production.
Membrane proteins are one example of antigens for which immunogens are difficult to prepare. Generally, membrane proteins are often difficult to highly express or sufficiently purify. These difficulties have been an obstacle in obtaining antibodies against membrane proteins.
Attention has been paid to methods that use baculoviruses to express large quantities of membrane proteins. By introducing a gene that encodes a subject membrane protein into a baculovirus genome, the subject membrane protein is expressed on the membrane surface of the budding baculovirus (WO 98/46777, Unexamined Published Japanese Patent Application No. (JP-A) 2001-333773). Using these methods enables expression of a large quantity of a subject membrane protein on a viral membrane surface.
However, in addition to exogenous membrane proteins, baculovirus-derived membrane proteins are also expressed on the membrane surface of the baculoviruses thus obtained. Thus, when budding baculoviruses are used as antigens, antibodies against baculovirus-derived membrane proteins may also be produced. Accordingly, it has been difficult to efficiently produce antibodies against subject membrane proteins by using known immunization methods.
For example, immunization using budding baculoviruses as antigens often induces antibodies that recognize gp64. The membrane proteins of baculoviruses comprise large quantities of gp64. In addition, due to gp64's high antigenicity, immunized animals can easily recognize gp64 as “nonself”. Consequently, budding baculoviruses can be thought to preferentially induce anti-gp64 antibodies.
Therefore, when using membrane proteins as antigens, the subject membrane proteins expressed on the baculovirus membrane surface must be sufficiently purified. However, purifying exogenous membrane proteins from budding baculoviruses is generally difficult. Thus, it can be said that sufficient quantities of highly purified membrane proteins cannot be practically obtained for use in immunization. Using conventional methods to obtain target antibodies for these difficult-to-purify antigens has been difficult.